Variable resistor with strain-reducing attachment means for the substrate

ABSTRACT

An adjustable resistor wherein a predetermined pattern of thin metallic film is deposited upon a substrate, a thin isolating layer being interposed between the substrate and film, and a movable contact system with suitable linear or rotary mechanical drive means is arranged for movability parallel to the substrate surface on which are fixed the isolating layer and metallic film, the contact system providing connection to the metallic film at a distance from one end thereof dependent on the position of the drive means. The pattern in the metallic film compels the electric current to flow through a path of limited width and of effective length much greater than the straightline length along the midline of the pattern.

United States Patent 2,597,674 5/1952 Robbins lnventor Felix ZandmanPhiladelphia, Pa.

Appl. No. 841,322

Filed July 14, 1969 Patented Aug. 24, 1971 Assignee VishayIntertechnology Inc.

Malvern, Pa.

VARIABLE RESISTOR WITH STRAIN-REDUCING ATTACHMENT MEANS FOR THESUBSTRATE 4 Claims, 6 Drawing Figs.

US. Cl 338/183, 338/292, 338/308 Int. CL 1101c 9/02 Field oiSelrch338/118,

Reterenees Cited UNITED STATES PATENTS 3,271,721 9/1966 Gordon 338/1803,405,381 10/ 1968 Zandman 338/308 FOREIGN PATENTS 406,634 5/1932 GreatBritain. 333/138 Primary Examiner--Lewis H. Myers AssistantExaminer-Gerald P. Tolin AttorneyThomas M. F erril, Jr.

ABSTRACT: An adjustable resistor wherein a predetermined pattern of thinmetallic film is deposited upon a substrate, a thin isolating layerbeing interposed between the substrate and film, and a movable contactsystem with suitable linear or rotary mechanical drive means is arrangedfor movability parallel to the substrate surface on which are fixed theisolating layer and metallic film, the contact system providing connection to the metallic film at a distance from one end thereof dependenton the position of the drive means. The pattern in the metallic filmcompels the electric current to flow through a path of limited width andof efiective length much greater than the straightline length along themidline of the pattern.

PATENTEDmszmn 3,601,744

I sum 1 ur 3 162 54 r 7 K4 ;////'////y//////// 141/ /1 .f/ I f/ INVENTORf/a'z Z animal? ATTORNEYS SHEET 2 BF 3 PATENTEB M1824 |97| INV NTQR fe/(x Za men/ 8 hkm z ATTORNEYS BACKGROUND OF THE INVENTION The presentinvention relates to electrical components and variable resistors. It isparticularly concerned with very stable variable resistors for use asrheostats and potentiometers.

Two prevalent types of variable resistors for use as rheostats orpotentiometers are wire-wound resistors and composition resistors. Insome examples of the former, a narrow, elongated card of fiberboard iswound with wire to the configuration of a flattened helix and is thencurved into an arcuate shape and mounted so as to have the resistivewire winding contacted by v a rotatable contact arm. For use of theapparatus as a rheostat,

one connection is made to one end of the resistive wire winding and theother connection is made to the rotatable contact arm. In order for theapparatus to be usable as a potentiometer resistor, an additionalconnection is provided to the other end of the wire winding. In someexamples of composition resistors, a resistive coating on a supportingcard is arranged to be contacted by a wiper, and connections are made tothe ends of the composition resistive element.

Both of these types of variable resistors have disadvantages. Thewire-wound resistors usually lack smoothness, mechanically andelectrically. Their resistance changes in a substantially step-by-stepfashion. Their wear life, while sometimes better than that of variableresistors with composition elements, is limited. Neither type resistoris readily adjusted to a precise predetermined function of resistanceversus control position. Each such unit is subject to appreciablevariation during manufacture. Each type provides resistance valuesgreatly dependent on ambient conditions including temperature.

It is an object of the present invention to provide a high precisionvariable resistor.

It is another object of the present invention to provide a very stablevariable resistor.

A further object is to provide a variable resistor wherein a desiredcharacteristic of resistance as a function of displacement of a movablecontact element can be achieved with great precision and be stablyretained throughout substantial changes of conditions including changesof temperature.

SUMMARY OF THE INVENTION In accordance with the present invention, animproved variable resistor is provided wherein the resistor elementconsists of a thin film of a resistive bulk metal, deposited upon a thinisolating layer of a dielectric such as a thin epoxy substance on thesurface of a substrate. The film, isolating layer and substrate arestrongly adherent to each other. A contact unit is arranged for movementalong a predetermined path, so that it is caused to establish electricalcontact with the thin film at any desired distance from one end thereofand thereby provide a variable resistance value between said end and thecontact unit. In order to use the variable resistor as a potentiometerresistor, it may be provided with a terminal for each end of theresistive film in addition to its contact unit terminal. Essentiallyzero temperature coefficient of resistance is obtained by proper choiceof coefficient of thermal expansion and thickness of substrate,isolating layer and bulk metal film, and temperature coefficient ofresistivity of the bulk metal film itself.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding ofthe invention, reference will be made in the following description tothe accompanying drawings, wherein:

FIGS. 1 and 2 are vertical and horizontal sectional views, respectively,of an embodiment of the present invention having a linearly movablecontactor;

FIG. 3 is a diagram of several alternative patterns for the re sistivemetal film;

FIG. 4 is a cross-sectional view of an embodiment of the presentinvention having a rotary contactor cooperating with resistive film on aflat substrate, the housing and control shaft being shown in section;

FIG. 5 is a sectional view of the embodiment of FIG. 4, taken on line5-5 thereof, and

FIG. 6 is a cross-sectional view of a further embodiment of the presentinventionhaving a rotary contactor cooperating with a resistive film ona substrate in a cylindrical arcuate configuration.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS OF THE INVENTION Referringnow to FIGS. 1 and 2, a high precision variable resistor is shown whichcomprises a housing 11, control knob 13, and a lead screw 15 arranged tobe rotated by means of the knob 13. Situated parallel to the lead screw15 is a substrate 17, supported as by cement bodies 16, 18. A coating 19which may comprise an epoxy resin or polyimide or equivalent is providedon the substrate 17. A metallic film 21 of resistive bulk metal issecurely held in place on'the substrate 17 by the adherent isolatinglayer 19 and is so arranged as to have its upper surface exposed inorder that it can be reliably contacted by a movable contact member.

A lead screw follower 25 is provided on the lead screw 15. Follower 25has a series of contact elements 27, 29; 31 and 33 carried thereby,preferably arranged on a resilient sheet metal member 35 which may, ifdesired, have the lower portion thereof slitted for maximum flexibilityand for independence of spring action of each of the contact elements.

The substrate 17 may be either a hard insulative substance such asglass, or may be a metal body, if desired. The coating layer 19 thereonholds the bulk'metallic film 21 in place upon the substrate 17,providing the necessary adhesion and electrical isolation to both themetallic film and substrate, whether the substrate is glass or metal oryet other material.

As described in US. Pat. No. 3,405,381, issued Oct. 8, 1968 to FelixZandman and Branin A. Boyd, the bulk metal film 21 may be made from aresistive alloy such as one of the Nichrome alloys, wherein nickel andchromium are the principal component metals. This film may be thin, forexample,'it may be of the order of 0.00002 to 0.0004 inches thick.

In order to provide a substantial value of resistance between the twoends of the metallic film 21, the current conduction path therein iscaused to have a length many times longer than the physical length ofthe substrate 17. One way in which this may be accomplished is byphotoetching the film 21 to a pattern such, for example, as the patternillustrated in FIG. 2. With the pattern as shown here, the current pathsfrom one end terminal junction 37 to the opposite end terminal junction39 involve traversals between the centerline of the film 21 and theouter edges thereof. The current is forced to flow through such paths bythe elongated, narrow gaps etched in the film 21. The first such gapsproceeding from the left end toward the right end of the film 21 are thegaps 41 and 41', between which the current is forced to flow through thenarrow middle portion of the film 21. Immediately to the right of theseelongated gaps 41 and 41' which extend out to the edge of the film 21 isa further elongated gap 43 which is bounded at both its ends by thefilm. Next to the right of this elongated gap 43 is a further pair ofgaps 45 and 45' extending to the edges of the film 21, at which thesegaps are open, and yet further to the right is another narrow elongatedgap 47 bounded at its ends by the film. This pattern is repeatedthroughout the length of the film 21 between the end portions thereof towhich are secured the end terminal junctions 37 and 39. The current pathfrom end terminal junction 37 extends approximately to the longitudinalcenterline of the metallic film 21 (between the mutually adjacent endsof gaps 41 and 41'), and extends outward toward the opposite edges ofthe film 21, around the ends of gap 43, and back toward the longitudinalcenterline, where the path continues between the mutually adjacent endsof gaps 45 and 45' and immediately extends back toward the oppositeedges of the film 21, around the ends of the gap 47.

Using this basic pattern, it is possible to achieve any desiredresistance value in a wide range by the selection of the thickness ofthe bulk metal film and the closeness of spacing (and consequently thenumber) of the elongated narrow gaps. The thinner the film, and thenarrower the film between gaps, in the pattern of FIG. 2, the higher theresistance value.

The contact elements 29, 31 and 33 are arranged to contact the film 21at any desired distance from the left end thereof,

according to the adjustment of the lead screw 15 and follower 25 bymeans of knob 13. Contact elements 29, 31 and 33 may be arranged in astraight line pattern as shown, or arranged in a slightly diagonalstraight line, if desired. If preferred, the middle element may bepositioned slightly the line of the other two. Also, if desired, thenumber of elements in contact with the film 21 may be greater or lessthan three.

Although it would be possible to arrange the lead screw and follower 25to provide the electrical path to the contact elements 29, 31 and 33through the spring means 35, it is preferable to provide circuit meansindependent of the frictional contact between the lead screw andfollower. For that purpose, a conductive strip 51 preferably is providedand arranged to be contacted by contact element 27 on one end of I theresilient metal member 35. The strip 51 may, if desired, be a narrowstrip of a bulk metal film deposited on substrate 17 with an epoxy layer19 between it and the substrate. Alternatively, the strip 51 may be ametallic strip otherwise secured upon, or merely arranged parallel to,the substrate 17, or even in a plane nonparallel to the substrate.

Terminal leads or lugs 53 and 55 are provided, connected to thejunctions 37 and 39, respectively, by internal connecting wires orribbons or pigtail connectors" 57 and 59. A similar terminal (notvisible in FIGS. 1 and 2) may be connected by a pigtail or ribbon 61 toa junction 63 on the conductive strip 51. The latter terminal isprovided for a circuit connection to the variable tap of the resistor.

Depending upon how exacting are the requirements for stability of thevariable resistor unit, the substrate 17 may be pro vided with a furthercoating on the bottom thereof substantially symmetrical with the coating19 which is relied on to isolate and hold firmly the film upon the topthereof. The way in In FIG. 33, none of the narrow elongated slits isshown extending to the edge of the metallic film, but the current pathsare compelled to extend through convolutions generally in the samemanner as those of FIG. 2. The pattern of FIG. 3B, by N virtue of theseveral incremental path portions effectively in parallel with eachother, provides a high degree of reliability and excellent immunitytoserious detrimental effect from any localized defect in or injury tothe film. It is of interest to note that with the pattern of FIG. 2 orthe pattern of FIG. 3B, just as with the aforedescribed pattern of FIG.3A, an adjustment maybe made in the graduation of the resistance withthe position of the traveller 25 by scribing out one or more gaps in thebulk metallic film, in a similar manner to the scribing illus- 3 tratedand explained in the aforementioned U.S. Pat. 'No.

3,405,381 to FelixZandman and Branin A Boyd. Thus, for example, the filmin the pattern shown in FIG. 38 may be scribed out to 'the edge alongthe dotted line 61, or scribed along dotted line 63 in the region of thecenterline of the film, or

which the stability of the resistive element is enhanced by the v Vuseof symmetrical layers of coatings such as epoxy coatings to inhibitbendingtendencies and control the related strains is explained in saidUS. Pat. No. 3,405,831.

FIG. 3 illustrates some of the alternative pattern configurations whichmay be used for the resistive film. The version illustrated in FIG. 3Acomprises the bulk metallic film having holes etched therein, such ascircular holes, for example. For a given thickness and width of thefilm, and a given number of holes, the diameter of the holes may bedecreased to provide a lower resistance value, or increased to provide agreater resistance value.

If desired the pattern of FIG. 3A may include arcuate gaps such as areshown in dotted lines at 58, 58'. Such arcuate gaps may be establishedin the regular course of making the patterned metal film, as by theirinclusion in a photographic master used for photoetching. Alternatively,the pattern may be photoetched without the arcuate gaps at the edges,and such gaps may be cut wherever needed in the course of trimming thepattern to make it coincide precisely with a predetermined resistancedistribution. Another type of trimming adjustment which may be relied onif desired is the cutting or scribing, by a stylus, for example, oflinear gaps extending outward to the edges from the circles in thepattern. Examples of this are illustrated in dotted lines at 60, 60' inFIG. 3A. To facilitate minute increases of the incremental resistancegradient at any desired point, a few very small openings such as squareopenings may be provided in the border of the metal film between the endof each linear gap and the edge of the film. This admits of scribing theintervening metal in one, two, or more places in line with any suchlinear gap according to the amount by which the incremental resistanceis to be increased.

scribed at a plurality of places chosen by the operator. With each suchscribing, the incremental resistance value per unit length along thesubstrate in the immediate vicinity of the scribed gap is increased.

FIG. 3C shows a pattern in which a longer path for current conductionmay be achieved in a given metallic film for the same number ofelongated gaps per unit length thereof. The current path in FIG. 3C,unlike the current paths illustrated in FIGS. 2, 3A and 3B, lacks thefeature of multiple mutual parallel incremental paths for redundancy ofthe current flow,

so that the version in FIG. 3C lacks plural paths substantiallysymmetrical about the longitudinal centerline of the film. One advantageof the use of the redundant path configurations is the addedprotectionagainst failure of the resistance in the event of a defect such as arupture or fissure in the metallic film. Another advantage of theillustrated configurations is the I minimization of the inductivecomponent, although all of the illustrated bulk metallic filmconfigurations provide extremely low inductive components, difiicult ifnot impossible to equal with wire-wound variable resistors.

FIG. 3D illustrates the arrangement of elongated slots or slits in thebulk metallic film in a somewhat slanted or slightly diagonal patternrelative to the centerline of the metallic film, the slanting beingexaggerated for clarity. Such a configuration may, if desired,- be usedto provide enhanced smoothness of the change of resistance with themovement of the adjustable traveller along the length of the substrate.

FIGS. '4 and 5 illustrate an embodiment of the invention in which thecontact member is angularly adjusted rather than longitudinally adjustedrelative to a substrate having a bulk metallic film established in apattern thereon.'The housing 71 is arranged to enclose a substrate 73upon which is deposited, and held in place by means of an epoxyisolating layer 75 (FIG. 5), a bulk metallic film 77 having a pattern ofslits or openings therein for causing current to flow in a tortuous patharound said film. A control shaft 79 and a knob 81 thereon are arrangedto provide for angular adjustment of a rotor arm 83 from which'depend aseries of contact elements 85, 87, 89 and 91. These contact elementspreferably are arranged upon a a resilient thin metal member 93extending downward from the arm 83, and preferably provided with slitsextending part way from the lower edge thereof up toward arm 83 in orderfor each of the contact elements 85, 87, 89 and 91 to be assured ofbeing held in contact with the fixed portion of the variable resistorsystem. The substrate 73 is supported from the base of the housing 71 bymeans of spaced supports such as those illustrated at 95 and 97, whichmay either comprise screwdown holding means or bosses provided withsuitable cement.

Terminal junctions 99, 101 and 103 may also be provided (FIG. 4), in thesame manner as the terminal junctions 37, 39 and 63 in the embodimentillustrated in FIGS. 1 and 2.

For the purpose of maintaining a stable, trouble-free connection to themovable wiping contact system carried upon rotor arm 83, a contact ring105 preferably is provided. Ring 105, which is arranged to be contactedby movable contact element 85, may, if desired, be a thin flat ring ofbulk metallic film fixed in place upon the epoxy layer 75 on the uppersurface of the substrate 73.

The bulk metallic film 77 (shown in FIG. 5) may be etched to flow inelongated paths. One example of such a pattern is illustrated in FIG. 4,this being a pattern having general similarity to the pattern shown inFIG. 2 in the first illustrated version of the present invention.Ifpreferred, the width of the slits may be tapered in order to have theedges of the conductive paths bounded thereby approximately parallel toeach other.

With respect to the structure shown in FIGS. 4 and 5, an epoxy layer maybe provided upon the opposite surface of the substrate for substantiallybalancing stresses exerted thereon, and thereby controlling the bendingstrains produced in the patterned metallic film.

FIG. 6 illustrates schematically in cross section yet a furthermodification of the present invention. In this embodiment, as in theembodiment of FIGS. 4 and 5, angular adjustment of the contact arm isused. In this version, the substrate 111 is not flat but is in the formof an arcuate segment of a cylinder. Accordingly, the contact arm 113carried upon shaft 115 may be provided with a series of contact elementssuch as element 117 spaced apart along a vertical line substantiallyparallel to the axis of the shaft 115. Such structure may be providedwithin a housing 119 which, if desired, may be a housing of any desiredshape, one example being a cylindrical housing similar to that shown inFIG. 6. I

In the structure illustrated in FIG. 6, the inside cylindrical wall ofthe substrate provides the support for the patterned bulk metal film. Aswill be apparent, this structure is adaptable to arrangements with thepatterned bulk metal film on the outer (convex) surface of the substrate111 having the form of an arcuate segment of a cylinder. In the lattercase, the angularly adjustable resistance control part is arranged toreach to the outer surface and contact the patterned film.

In any of the illustrated structures, metal film paths for electriccurrent may be provided on both the opposite surfaces of the substrate.As one example, a tortuous high-resistance path may be provided on onesurfaceand a lower resistance path for establishing contact for theadjustable means may be provided on the opposite surface.

In making the structure illustrated in FIG. 6, one may prefer toestablish the resistance pattern in the bulk metal film while it isflat, as by photoetching it. If so, the substrate 111 may initially beflat and may be retained so until after the pattern has been establishedin the bulk metal film thereon, and thereafter the substrate 7 may becurved into its cylindrical arcuate geometry. Such a flexible substratemay be made of a thin aluminum sheet or steel sheet and the epoxy layerprovides isolation and holds in place the bulk metal film.

The bulk metal film and the substrate conform to I-lookes law, and otherclassic properties of elastic bodies subjected to stress. Accordingly,in the variable resistors of the present invention, as in fixedresistors having bulk metal film upon a substrate, the response tochanges of conditions such as temperature changes can be determined bytaking into account the expansion characteristics and modulus ofelasticity and thickness of each of the layers of material including notonly the substrate and the bulk metal film but also the dielectricadhesive layer. The layer provides for faithful strain transmission fromsubstrate surface to metal film and the layer is substantially free ofcreep at any normal operating temperature.

In the manufacture of the substrate-supported resistive element includedin the present invention, the bulk metal film may be deposited on thesubstrate and may thereafter be photoetched to the desired pattern toachieve the desired substantial resistance value between the endsthereof. Altema' tively, the bulk metal film may be supported on acarrier such as a flexible or semiflexible sheet of material until afterthe film has been photoetched to the desired pattern, and it maythereafter be transferred to the substrate to be cement bonded thereonbefore removal of the carrier sheet. It is also feasible to retain thecarrier sheet, if it is made of a dielectric, bonding it to thesubstrate and thus having it kept in place between the bulk metal filmand the substrate.

In each of the embodiments shown, the end portions of the patternedmetal film may be rendered more secure by epoxy imbedment. Suchreinforcement may also be added along the sides of the pattern ifdesired, the surface of the patterned metal film remaining exposed inthose areas through which the contact elements are movable.

In the foregoing description, it has been explained that finishingadjustments of the resistance gradient with respect to contactorposition along the film pattern can be performed by cutting or scribingslits or other gaps in portions of the film pattern. Alternatively, anyportions of the pattern where an increased resistance gradient isdesired may be rubbed with a fine abrasive such as pumice powder, or maybe rubbed with an etchant, or may be subjected to electroetching. By anyof these procedures, or combinations of selected ones, the resistancecharacteristics may be tailored to a desired pattern.

In addition to the end terminals shown, it will be recognized that fixedintermediate resistance taps may be included if desired.

As to the mode of attachment of the substrate in the housing, one ormore of the illustrated bodies of cement may be made of material havinga low modulus of elasticity or the cementing with high modulus cementsis made so as to minimize any strains induced in the substrate bydifferences of expansion characteristics of the substrate and thehousing, in response to temperature changes, for example. Such materialas-a polyurethane plastic may be used to provide the degree offlexibility needed, or a low modulus epoxy may be used. High moduluscement can be used if attachment is made on one end only so as to permitrelatively free expansion of the substrate.

Features of the adjustable resistor described herein include itssuperior resolution and great reliability, especially with the patternscharacterized by redundant current paths. Further features attainableare the minimization of the dependence on temperature, a substantialtemperature coefficient of re sistance being avoided unless othercircuit factors make it desirable to do otherwise. Typically, atemperature coefficient of resistance of :3 parts per million per C.over the range from 55 C. to+l25 C. is attainable. One way of attaininga high temperature coefficient of resistance where such is needed for aspecial circuit is by mismatching the coefficients of thermal expansionof the substrate with the characteristics of the metallic film and theisolation layer. With these features, in addition to its low electricaland mechanical noise characteristics and its suitability for a widefrequency range by virtue of its very low inductance and verylowdistributed capacitance, the adjustable resistor of the presentinvention is particularly suited for a variety of precisionpotentiometer, rheostat and bridge circuit applications.

As many changes may be made and many widely different embodiments may beconstructed without departing from the spirit of the present invention,the accompanying drawings and description are intended not by way oflimitation but rather illustration of forms which may be taken by theinvention, the scope of the invention being indicated by the claims.

What I claim is:

1. An adjustable electrical resistor device having a temperaturecoefiicient of resistance within :3 parts per million per C. over therange of -55 C. to +l25 C., which resistor device consists essentiallyof:

a substrate;

an isolating layer of a dielectric on one face of said substrate, saidisolating layer being substantially free of creep under normal operatingtemperatures;

a metallic film 0.00002 to 0.0004 inches thick, made from resistive bulkmetal held in place adjacent to one face of the substrate by theisolating layer which provides faithful strain transmission from thesubstrate to the metallic film, wherein said metallic film is providedwith openings for restricting and rendering tortuous the conductive path7 8 therein and thereby impeding the flow of electrical curduced in thesubstrate by differences of expansion rent from one end of the metallicfilm to the opposite end characteristics of the substrate and housing,said nonconthereof; ducting attachment means contacting only a smallarea of a conductive strip held in place on one face of the substratethe substrate and said nonconducting attachment means by the isolatinglayer adjacent to the metallic film without being noncontinuous betweenthe substrate and the houscontacting the metallic film; ing.

a multifingered movable contact means for contacting the The adjustablel l re r device Of claim 1 conductive trip and the metallic and therebywherein the attachment means interposed between the subestablishing anelectrical connection between the con- Straw and the housing has 310Wmodulus of elastisitl ductive strip and the metallic fil at a variabledistance 10 3. The adjustable electrical resistor device of claim 2 fone end f the resistor device; wherein the attachment means are presentat opposite ends of a housing for the adjustable electrical resistordevice; the substrate-.

a shaft rotatably mounted in said housing, said shaft having Tadjustable electrical resistor device of claim 1 a threaded section forengaging and moving the multifin- 'fl the attachment means a h modulusof gered movable Contact means; and elasticity and are attached only toone end of the substrate so nonconducfing attachment means interposedbetween the as to prevent relatively free expansion of the substratesubstrate and the housing which minimizes strains in-

1. An adjustable electrical resistor device having a temperaturecoefficient of resistance within + OR - 3 parts per million per * C.over the range of -55* C. to +125* C., which resistor device consistsessentially of: a substrate; an isolating layer of a dielectric on oneface of said substrate, said isolating layer being substantially free ofcreep under normal operating temperatures; a metallic film 0.00002 to0.0004 inches thick, made from resistive bulk metal held in placeadjacent to one face of the substrate by the isolating layer whichprovides faithful strain transmission from the substrate to the metallicfilm, wherein said metallic film is provided with openings forrestricting and rendering tortuous the conductive path therein andthereby impeding the flow of electrical current from one end of themetallic film to the opposite end thereof; a conductive strip held inplace on one face of the substrate by the isolating layer adjacent tothe metallic film without contacting the metallic film; a multifingeredmovable contact means for contacting the conductive strip and themetallic film and thereby establishing an electrical connection betweenthe conductive strip and the metallic film at a variable distance fromone end of the resistor device; a housing for the adjustable electricalresistor device; a shaft rotatably mounted in said housing, said shafthaving a threaded section for engaging and moving the multifingeredmovable contact means; and nonconducting attachment means interposedbetween the substrate and the housing which minimizes strains induced inthe substrate by differences of expansion characteristics of thesubstrate and housing, said nonconducting attachment means contactingonly a small area of the substrate and said nonconducting attachmentmeans being noncontinuous between the substrate and the housing.
 2. Theadjustable electrical resistor device of claim 1 wherein the attachmentmeans interposed between the substrate and the housing has a low modulusof elasticity.
 3. The adjustable electrical resistor device of claim 2wherein the attachment means are present at opposite ends of thesubstrate.
 4. The adjustable electrical resistor device of claim 1wherein the attachment means have a high modulus of elasticity and areattached only to one end of the substrate so as to prevent relativelyfree expansion of the substrate.